Kingdom of Cages by Sarah Zettel

This novel takes place in a distant future in which humans have left Earth-after ecologically destroying it-and made their homes among distant planets. Many generations later a plague is destroying human life on the colonies and space stations. Scientists can not figure out what is wrong, in desperation the colonists seek help from the earth they left behind. Earth is happy to help until the plague begins to spread to earth. The colonies are desperate and turn to Pandora-a colony of anal retentive people who live in on the most earth like planet-in a bubble so they can study the planets naturaly ecology and not make any changes or damages. They have no plague on their world at all, but they do not allow outsiders access. The colony fleet gives an ultimatum, help us cure the disease or we destroy your pristine planet. Pandora in desperation-has their advanced scientists and technicians begin work on the problem immediately.
Twenty years with no cure in site and under constant threat from the colony fleet, the Pandorans find a family with the genetic markers they need to complete their cure Chena and Teal Trust will never be the same again.
~~~~Where ever you go, There you are!